1. Field of the Invention
The present invention relates to a container of biaxially oriented crystalline resin, particularly, having an improved bottom portion, and a process of making the same.
2. Description of the Related Art
Containers made of biaxially oriented crystalline resins such as polyethylene terephthalate (PET) have many advantages in gas-barrier property, transparency, toughness, hygiene and others.
It is normally required that the containers of PET are formed to resist an impact and a rapid increase of internal pressure due to such an impact when they are dropped onto a hard surface from at least a substantial height under such a condition that the containers are filled with liquid and capped. The containers of PET are frequently formed to have one of the most conventional bottom structures taken in glass bottles, that is, a bottom structure similar to that of a champagne bottle. The external surface of such a bottom structure generally includes a central recessed portion and a marginal raised portion surrounding the central recessed portion and gradually extending outwardly and axially from the central recessed portion, the marginal raised portion transiting to the lower end of the side wall of the container. The lowermost portion of the marginal raised portion is entirely in a planer plane such that the container can stand upright on the horizontal plane.
When a container having its bottom similar to that or the champagne bottle is to be formed of biaxially oriented PET, the bottom of such a container may have a wide portion which has not been molecular oriented. Such a not-oriented bottom portion is poor in heat-resistance. If the container is filled with a high temperature liquid, therefore, the central recessed portion of the bottom thereof will be deformed outwardly, resulting in loss of the self-supporting property.
Many proposals have been made which improve the heat-resistance in the bottoms of PET bottles used to charge liquid at raised temperatures. Some of such proposals are disclosed, for example, in Japanese Patent Laid-Open Nos. Sh. 62-146137, Sho 53-108174 and Sho 60-172636, Japanese Patent Publication No. Sho 57-57330, and U.S. Pat. No. 3,468,443. Particularly, Japanese Patent Laid-Open No. Sho 62-146137 discloses a technique of arranging a plurality of radial ribs on the bottom portion of a container to promote the orientation and crystallization in the bottom portion for improving the heat-resistance.
The complicated arrangement of the ribs on the bottom of the PET container degrades the appearance of the container itself and also requires a very expensive blow mold having a cavity of complicated configuration.
A technique other than the above techniques also is known which is to improve the heat-resistance in the aforementioned type of PET containers by crystallizing all the bottom portion of a PET container into spherulite-like crystals. However, such a technique certainly improves the heat-resistance in the PET containers, but raises another problem in that a PET container produced according to this technique tends to crack at a position adjacent to the gate of the central bottom portion when it is dropped onto the hard surface.
In order to provide spherulite-like crystals over all the bottom of the PET container, the latter must be heated such that the central thick-walled bottom portion having a higher heat capacity will be heated up to a temperature higher than a predetermined temperature of crystallization. Such a heating process requires a prolonged time, resulting in very reduction of the producibility.